1. Field of the Invention
The present invention relates to a hydraulic control system for an transmission used in vehicles.
2. Description of Related Art
Generally, a conventional automatic transmission includes a torque convertor, a multi-stage gear shift mechanism connected to the torque convertor, and a hydraulic control system which selects a gear stage of the gear shift mechanism according to a driving state of the vehicle.
The above hydraulic control system includes a pressure regulator, which regulates hydraulic pressure created in a hydraulic source; manual and automatic shift controllers, which select a shift mode; a hydraulic pressure controller, which controls shift quality and shift response for smoothly selecting a shift mode during shifting; a damper clutch controller, for operating a torque convertor damper clutch; and a hydraulic pressure distributor, which supplies an appropriate amount of hydraulic pressure to each of the frictional elements.
In such a hydraulic control system, according to operations of solenoid valves which are On/Off or duty-controlled by a transmission control unit, the pressure distribution by the hydraulic pressure distributor is varied to operate frictional elements associated with respective speed ratios.
When controlling a shift operation from a one speed ratio to another, a timing between exhaust of hydraulic pressure and supply of hydraulic pressure to a new set of frictional elements to shift into another speed ratio affects shift quality.
If the timing is not precise, engine RPM may abruptly increases or the shift mechanism may lock.
Therefore, to precisely control the timing, a very complicated control system has been used.